Advances in Electrical and Electronic Engineering
98
SOLUTION CONCEPT OF MODULAR SINGLE PHASE ACTIVE POWER
FILTERS
M. Roch, P. Braciník, A. Hrebe ár
University of Žilina, Faculty of Electrical Engineering, Univerzitná 1, Žilina 010 01, Slovakia
Slovenská elektriza ná prenosová sústava a.s., Poštová 2, Žilina 010 08, Slovakia
e-mail: Marek.Roch@kves.utc.sk, Peter.Bracinik@kves.utc.sk, Hrebenar_Anton@sepsas.sk
Summary This paper investigates a modular or a decentralised single-phase active power filter control strategy. It is based
on the evaluation of the harmonic reference load currents for the active power filter blocks operating under specific harmonic
frequencies. The underlying principle of the modular active power filter is explained and it is shown how the required
reference harmonic currents can be evaluated. Simulation results demonstrated the improvement in the dynamic performance
of the modular active power filter presented here in comparison with the conventional type.
1. INTRODUCTION
Electrical power systems loads usually include
non-linear loads such as solid state power
converters. These converters normally operate in the
switching mode. Therefore, they result in the
generation of higher harmonic current in the
electrical power transmission system. It can cause
considerable losses and voltage distortion,
electromagnetic
equipment
failure
and
electromagnetic interference to neighbourhood
appliances and inefficient use of electric energy. In
addition, reactive currents are drawn from the ac
mains due to the reactive nature of the non-linear
loads. These reactive currents lead to low power
factor operation of the electric power system.
Passive filters are traditionally used to filter out
the voltage and current harmonics in the power
systems. Normally, they can not compensate for the
low power factor. In addition, they are inefficient
because the switching frequency of the non-linear
load power converters varies and can reach high
values. Moreover, the passive filters are bulky and
expensive.
Active power filters represent more effective
solution to the power system quality issue. Their
goal is to improve both of the harmonic distortion
and the power factor of the current drawn from the
ac mains at a fraction of the cost of the passive
filters.
An active power filter basically consists of a
power electronic converter and a source of electric
energy. The latter represents the active part of the
filter. It enables the filter to provide any variations
of the switching frequency of the converter.
Modular active filters are capable of filtering
out the voltages and currents of higher harmonics at
the vicinity of their generation. Hence, it ensures
that these harmonics are not transmitted throughout
the distribution electrical power system. However,
the active power filters can not eliminate the higher
switching frequency harmonics caused by the power
electronic converter constituting these filters.
The switching frequency is kept constant and
therefore, these harmonics can be removed by using
specially designed passive filter.
Since active power filters compensate for both
of the reactive and harmonic current components in
the electric power systems, they give rise to
considerable energy and cost savings.
Active power filters which has been reported
[1], [3], compensate for the power factor and for the
lower current harmonics only (up to the 5th order).
This is largely due to the limitation of the
computational capacity of the microcomputers used
to implement the control strategies necessary for
these filters and due to the maximum amount of
switching losses that the power electronic devices
used in the power converters comprising the filters
can cope with.
Modular active power filters reported in the
present work are based on the separate computation
and implementation of the control strategies to
evaluate the reference currents for individual
modules of the modular filter. These modules
operate at the fraction of the rating of the modular
filter, compensate for a specific harmonic frequency
and carry their own individual computational
facilities. Therefore, modular active power filters are
capable of compensating higher order harmonic
currents than the singular structure ones. Additional
advantages of these modular filters are enhanced
reliability and increased operation efficiency. The
first is due to the fact that the failures of one module
in these filters will neither adversely affect the
operation of the modular filter nor will it bring to a
complete halt. The other is due to the smaller
switching loss of each module in the filter because
of the reduced power-handling requirement of each
module. This leads to a reduction in the total
switching loss in the modular filter in comparison to
the singular one. One additional advantage of the
modular filters is their improved transient response
in comparison to singular filters. Obviously, this
improvement is highly dependent on the selected
control strategy to evaluate the required harmonic
reference currents.
The block diagram of a modular active power
Solution concept of modular single phase active power filters
99
filter is shown in Fig. 1. SVC (Static Var
Compensator) generates the reactive reference
current component. APF3 (active power factor
module for the third harmonic) generates the third
harmonic of the reference load current. APF5 (active
power factor module for the fifth harmonic)
generates the fifth harmonic of the reference load
current. The modules APF7, APF9, etc., generate the
other higher harmonics of the reference load current.
Each of these modules comprises a power converter
(usually built of IGBT solid state devices), a
harmonic analyser and a passive low pass filter. The
harmonic analyser contain mainly an analog to
digital converter, a digital to analog converter and a
digital signal processor to carry out the necessary
computations to evaluate the harmonic reference
current. The low pass filter compensates the
switching frequency signal caused by the switching
action of the power converter.
Supply
voltage
iloa d current
sensor
Modular filter current
isource
Nonlinear
load
Low
pass
filters
Harmonic
refrence
currents
Reacti ve
reference
current
IGBT
Power
Conv.
SVC
Active
power filter
module for
3 rdharmonic
(APF3)
IGBT
Power
Conv.
Active power
filter module
for 5 th
harmonic
(APF 5)
th
Modules for 7 and
9 th et c active power
filt ers
Harmonic
analyser
(HA5)
Harmonic
analyser
(HA 3)
Main(mast er)
feed back
current
loop
SCS
Volt age feed
forward loop
i load
ireference
Fig. 1. Block diagram of a modular active power filter
It is obvious that the modular active power filter
consists of a number of modules of active power
filters where each module is tuned to generate
a specific frequency of the reference harmonic
current.
The SCS (Supervisory Control System)
oversees the operation of all above mentioned
modules and takes appropriate action to compensate
the failure of any module.
2. PRINCIPLE OF OPERATION
Fig. 2 illustrates the control strategy adapted for
the modular active power filter of Fig. 1. There are
two levels of control systems. The algorithms in
these control systems are executed simultaneously
with the aid of parallel computation algorithms.
ifilter
isou rce
iloa d
N on-linear
load
Modula r active
powe r filter
1st harmonic
analyse r or
static var comp
IGBT powe r
c onve rter
IGBT power
converter
rd
3 harmonic
module of the
filter
7 th , 9 th harmonic
module, etc
rd
3 ha rmonic
a na lyser
th
5 harmonic
analyse r
5t h ha rmonic
module of the
filter
Supe rvisory control
system (SCS)
Level 1 control system
Level 2 control system
Fig. 2. Control system levels of the modular active power
filter
Level 1 control system carries out the following
tasks: computing the first harmonic of the load
current for power factor compensation, performing
the main (master) control loop algorithms, and
supervising the successful operation of the filter
modules for higher harmonics and taking
appropriate action in case of their failures.
Level 2 control system computes the higher
load current harmonics (3rd, 5th, 7th, etc.) and
provides local slave current loop control.
It is important to point out that both levels (1
and 2) of control systems operate synchronously.
In order to compute the fundamental and higher
load current harmonics Fourier harmonic analysis is
carried out in the stationary (α, β) frame of
reference. This implies that in the case of the singlephase power system being under consideration, the
frequency complex domain trajectory has a 4 sided
symmetry shape. Whereas the three-phase supply
has a 6 sided symmetry shape. The mathematical
procedure for evaluation of the reference currents
harmonics is explained in the next section.
3. EVALUATION OF THE
CURRENTS HARMONICS
REFERENCE
The orthogonal transform technique used for the
three phase power system in reference [8] is
modified to be suitable for the single-phase power
system under consideration in this paper.
The harmonic function describing either of the
power system voltage or current is complemented by
its orthogonal component so as to form a complex
exponential function in the complex domain:
cos(ωt)
exp ( j ωt) = cos(ωt) + j sin(ωt).
(1)
This approach can be extended to non-harmonic
periodic functions, accordingly:
cos(ωt + ϕ)
exp(j ωt + ϕ).
(2)
Advances in Electrical and Electronic Engineering
Based on the four-side symmetry of the trajectories
in the complex plan, the orthogonal co-ordinates of
the non-harmonic periodic voltages and currents can
be generally defined by the following complex
function:
X(j ωt) = K[xα(t) + xβ(t) exp( jπ/2) ].
(3)
Thus, for K=1, the complex function (representing
either a voltage or a current) can be decomposed to
its α and β components in the stationary frame of
reference as follows:
- for voltages:
uα = u (t)
and
uβ = u(t-T/4),
(4)
where T is the periodic time,
- for currents:
iα=i (t)
and
iβ=i(t-T/4).
(5)
It is noted that the β components in both of
equations (4) and (5) are fictitious components and
the used orthogonal transformation technique can be
extended to any other physical quantities in the
power systems.
The fictitious β components are used to
facilitate the computation of the reactive and
harmonic current components in a single-phase
power system as it is explained below.
The complex Fourier coefficients for the nth
harmonic of a non-harmonic, periodic quantity, x (t)
can be evaluated as follows:
Cn =
4
T
T
4
x(t ) e-jnωt dt.
100
Equation 10 in conjunction with equations (8) and
(9) are used to compute the values of the reference
harmonic load currents in the harmonic analysis
blocks of Fig.1 and Fig. 2.
4. SIMULATION RESULTS
Using Matlab power block tool, simulation
results for the modular active filter of Fig. 1 and
Fig. 2, and for the conventional type active power
filter of reference [4], [5], [7], were generated.
The results are simultaneously shown in Fig. 3(a)
and Fig. 3(b).
As it can be seen from these figures, for the
modular filter discussed in this paper, the power
system source current reaches, starting from the
shown dotted line, its quasi-sinusoidal time varying
state after the elapse of approximately the quarter of
the periodic time of the load current. While for the
conventional active power filter it takes the source
current about one period to reach its steady state.
Thus, the modular filter has improved dynamic
performance compared to the conventional active
power filter.
(6)
0
Using equation (3) for K=1, its possible to calculate
the α and β components of Cn, thus:
4
T
T
4
Cnβ=
T
T
Cnα=
(xα(t) cos(nωt) + xβ(t) sin(nωt)) dt,
(7)
(xβ(t) cos(nωt) - xα(t) sin(nωt)) dt.
(8)
0
0
According to equations (7) and (8), the
magnitude and phase shift of an nth harmonic
component of any non-harmonic periodic quantity
x (t) can be evaluated as follows:
Cn = (C2nα + C2nβ)1/2 and ϕn = arctan(Cnα/Cnβ).
(9)
Thus, ignoring the fictitious component,
harmonic of x (t) can be computed as follows:
nth
xn(t) = Cn cos ϕn cos(nωt).
(10)
Fig. 3. Current transients under distributed control using
parallel computation algorithms a) and under
classical control b)
It is important to mention that the solid state
power devices that are available nowadays and
which are used in the power converters comprising
the active power filters can carry currents of very
high capacities. This implies that the improved
dynamic feature of the modular filter described here
might not seem to be of a major significance.
However, the transient voltages induced across these
devices, due to the parasitic inductances of the
101
Solution concept of modular single phase active power filters
power supply and the transformers, is a major cause
for failures of these devices. Hence, the proposed
modular filter in this paper offers a solution to
overcome this problem.
Fig. 4. Experimental results under distributed control
using parallel computation algorithms with RL
load a) and with RC load b)
Experimental verification of the simulation
results for the modular active power filter are shown
in Fig. 4(a) and Fig. 4(b). Computations of the
reactive and reference currents and the necessary
control algorithms would be carried out using of the
Texas instruments, TMS320C31 floating point DSP.
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